The present invention relates to a circuit arrangement for the operation of at least one discharge lamp with an input for connecting an AC supply voltage, a rectifier, which is coupled to the input and has a first output terminal and a second output terminal for providing a DC supply voltage, an inverter, which includes at least a first switch and a second switch, which are coupled in series between the first output terminal and the second output terminal of the rectifier, a control circuit which is at least designed to drive the first switch and the second switch, with a first input for receiving a control signal, and a second input for feeding a voltage for supplying the control circuit, the second input being connected to an operational supply apparatus, which is designed to couple an operational supply voltage to the second input of the control circuit during operation of the inverter and a control line, which is coupled firstly to the input for connecting an AC supply voltage and secondly to the first input for receiving a control signal, a third switch being arranged in the control line. The invention moreover relates to a method for the operation of at least one discharge lamp using such a circuit arrangement.
Circuit arrangements, in particular electronic ballasts, which are switched on and off via a control input, have a power consumption in the switched-off state of up to 1 W, which is referred to as standby losses. The cause of these losses is primarily the internal control circuit, which needs to be supplied with voltage even in the “off” state, in order to be able to react to the next “on” command. The losses caused by the internal control circuit make up approximately 60% of the standby losses. 30% of the standby losses are produced by the filament monitoring, and 10% of the losses by the switched-mode power supply.
For a more detailed illustration of the problem on which the invention is based, reference is first made to FIG. 1, which shows a circuit arrangement of the generic type known from the prior art. Said circuit arrangement has an input E1, E2 for connecting an AC supply voltage UN. Connected thereto are a first X capacitor Cx1, two inductances L1a, L1b, which are coupled to one another, a rectifier 10 and a second X capacitor Cx2. A discharge lamp La is connected via an inductance L3 to the half-bridge center point HM of an inverter, which includes a first switch Q2 and a second switch Q3. A capacitor C3 is arranged in parallel with the lamp La. The circuit arrangement includes a control circuit 12, which in this case solves various problems. Firstly, the first switch Q2 of the inverter is driven via a control output HSG of the control circuit 12, and the switch Q3 of the inverter is driven via a control output LSG of the control circuit 12. An input Line is used for measuring the system voltage, which input evaluates the voltage across a nonreactive resistor R2 of a voltage divider including the nonreactive resistors R1 and R2. The resistor R1 is coupled to the voltage provided at the capacitor Cx2. Using the inductances L2a, L2b, which are coupled to one another, and the nonreactive resistor R8, a zero crossing of the current is determined by the inductance L2a via an input ZC in order to drive a switched-mode power supply, which includes a transistor Q1, a diode D1, the inductances L2a, L2b, a nonreactive resistor R8 and a capacitor C1, in a suitable manner via an output PFG. In this case, the transistor Q1, the diode D1 and the capacitor C1 form a PFC (power factor correction) apparatus. Using the nonreactive resistors R3 and R4, which together form a voltage divider, the amplitude of the so-called intermediate circuit voltage UZW is measured via an input Bus. The clock signal produced by a pushbutton T1 which in this case is coupled to the input E2, is converted via a pulse-shaping apparatus 14, which includes the nonreactive resistors R9 and R10 and a diode D4 and a capacitor C6, into a pulse, which is coupled to the input Cntrl of the control circuit 12 via the section 16 of a control line St. Via a nonreactive resistor R7, which is coupled to the intermediate circuit voltage UZW, the control circuit 12, after reception of an off signal at the Cntrl input, continues to be supplied with voltage via the input SS in order to be reception-ready for the next on command at the Cntrl input. During operation of the discharge lamp La, the control circuit 12 is supplied with voltage via an operational supply apparatus, the operational supply apparatus including the capacitor C2, the diodes D2 and D3 and the capacitor C5. In this case, the voltage UVCC is provided across the capacitor C5, which is coupled to the input VCC of the control circuit 12. This input VCC is furthermore used for determining, using the nonreactive resistors R5 and R6 and the capacitor C4, whether a discharge lamp La has been inserted.
Details regarding operation: The circuit arrangement illustrated in FIG. 1 is part of an electronic ballast, which can be switched on and off via a pushbutton T1. In this case, the system voltage UN is present across the circuit arrangement even in the off state. It is only the internal control circuit 12 which is stopped, with the result that the PFC transistor Q1 and the inverter, which includes the switches Q2 and Q3, are no longer driven. The internal control circuit 12 needs to continue to be supplied with voltage via the nonreactive resistor R7 in order to be reception-ready for the next on command. The internal control circuit 12 in this case, together with its supply via the nonreactive resistor R7, causes the majority of the standby losses.
Furthermore, a plurality of current paths which produce the measured values for the various functions of the electronic ballast increase the standby losses: Particular mention should be made of the filament monitoring, which includes the two nonreactive resistors R5 and R6, which are connected in series with an electrode of the fluorescent lamp La and only allows a current to flow from the so-called intermediate circuit, i.e. the line of the circuit arrangement across which the intermediate circuit voltage UZW is provided, to the internal control circuit 12 when the fluorescent lamp La is inserted. The filament monitoring ensures that the electronic ballast can start only when the fluorescent lamp La is inserted, that the electronic ballast, which is automatically shut down once a lamp fault has been identified, remains shut down, and that the electronic ballast automatically restarts (relamping) once a defective fluorescent lamp has been replaced.
The switched-mode power supply, which includes the inductances L2a and L2b, the nonreactive resistor R8 and the PFC apparatus, which has the transistor Q1, the diode D1 and the capacitor C1, is controlled in such a way that the intermediate circuit voltage UZW is kept at a constant value in a load-independent manner, and that the system current drawn is sinusoidal. For this purpose, a control signal is used which is proportional to the product of the control discrepancy between the intermediate circuit voltage UZW and the present value of the system voltage UN. For this purpose, the system voltage UN is interrogated via the nonreactive resistors R1 and R2, and the intermediate circuit voltage UZW is interrogated via the nonreactive resistors R3 and R4, by the internal control circuit 12.